1. Tryptophan synthase. We have continued work on the high resolution structure of the bifunctional enzyme complex tryptophan synthase. This complex is a paradigm for the study of intersubunit communication and allosteric activity. We have examined the mutant K87T which exists as the external aldimine combined with serine and with tryptophan. A series of conformational differences have been observed that provide a basis for understanding the mechanism of action and the communication between the two active sites. 2. Deoxyhypusine synthase. The crystal structure of the enzyme Deoxyhypusine synthase has been determined at 2.2A resolution. The enzyme is a homotetramer consisting of two pairs of closely linked subunitswith the active sites shared between the subunits. the active site is located in a deep cavity in which the spermidine is presumably bound. The nicotinamide ring is oriented in a manner consistent with the established stereospecificity of the hydride reaction. One interesting feature of the structure is that there is an N-terminal helix from a third subunit that forms a ball and chain arrangement which is located so as to block access to the active site. 3. HP1 structure. The structure of a dimer of the catalytic domain of the site-specific integrase from the HP1 bacteriophage has been determined to 2.5A resolution. The structure demonstrates that the four invariant catalytic amino acids at the active site are contributed by one monomer. The placement of the catalytic tyrosine (the nucleophile which attacks the phosphodiester backbone) and a highly positively charged cleft on the surface of the protein allow us to model the binding of DNA to the active site.